The present invention relates to pulverized coal-fired furnaces and, more particularly, to an apparatus for tilting a vertical array of fuel-air admission assemblies, at least one of which employs a split coal bucket for improved low load firing.
One method of firing coal in conventional coal-fired steam generator boilers is known as tangential firing. In this method, pulverized coal is introduced to the furnace in a primary air stream through burners, termed fuel-air admission assemblies, disposed in windboxes located in the corners of the furnace. Each windbox comprises a vertical array of alternate secondary air compartments and fuel-air admission assemblies. The fuel-air streams discharged from these burners are aimed tangentially to an imaginary circle in the middle of the furnace. This creates a fireball which serves as a continuous source of ignition for the incoming coal. More specifically, a flame is established at one corner which in turn supplies the required ignition energy to stabilize the flame emanating from a corner downstream of and laterally adjacent to it. However, at low loads the fireball deteriorates to four individual flames which frequently do not interact sufficiently to stabilize each other. Thus, at low loads it was frequently necessary to use auxiliary fuel such as light oil or natural gas to provide stabilization.
A distinct advantage of the tangential firing concept is that a wide range control of steam temperature can be obtained by tilting in unison the auxiliary air compartments and the fuel-air admission assemblies of the individual windbox upward or downward. By so doing, the fireball is physically raised or lowered within the furnace so as to increase or decrease the heat absorption by the furnace bounding waterwalls thereby effecting wide range control over the temperature of the combustion gases leaving the combustion zone and passing over downstream superheat and reheat surfaces. By tilting the fuel-air admission assemblies upward as load decreases, low load operation can be achieved while holding the overall cycle efficiency and maintaining better operation of the turbine. Additionally, the vertical adjustability of fuel-air admission assemblies permits the operator of the furnace to compensate for changes in heat absorption within a furnace waterwall resulting from fuel variations, in particular, for variations in the amount of slagging of the furnace waterwalls between different coals.
A recent improvement in the low load operation of coal-fired furnaces is disclosed in U.S. Pat. No. 4,252,069 issued Feb. 24, 1981. This patent discloses an improved fuel-air admission assembly incorporating a split coal bucket which permits a pulverized coal-fired furnace employing a tangential method to be operated at low loads without the use of auxiliary fuel to provide stabilization.
The low load fuel-air admission assembly disclosed in U.S. Pat. No. 4,252,069 comprises a split coal bucket having independently tiltable upper and lower coal nozzles pivotally mounted to the coal delivery pipe. When the furnace is operating at low load such as during minimum demand periods, the primary air and pulverized coal streams discharging from the coal delivery pipe are split into an upper and lower coal-air stream and independently directed into the furnace by tilting the nozzles away from each other. In so doing, an ignition stabilizing pocket is established in a locally low pressure zone created between the spread apart cool-air streams. Hot combustion products are drawn, i.e., recirculated, into this low pressure zone thus providing enough additional ignition energy to the incoming fuel to stabilize the flame and eliminating the need for auxiliary stabilizing fuel such as oil or natural gas.
U.S. Pat. No. 4,304,196, issued Dec. 8, 1981, discloses a tilt apparatus for use in conjunction with a windbox incorporating such a low load, split bucket fuel-air admission assembly. In accordance with this patent two completely independent tilt linkage means are provided, the first to link the single nozzle coal buckets and the upper nozzle of the split buckets together so as to tilt in unison and the second to link only the lower nozzles of the split coal buckets together so as to tilt in unison. Further, two independent tilt actuation means are provided, the first to actuate the first tilt linkage means, the second to actuate the second tilt linkage means. The first tilt actuation means which is associated with the first tilt linkage means is responsive solely to steam temperature, while the second tilt linkage means is responsive to steam temperature only at high loads and to load level at low loads.
It is an object of the present invention to provide an improved apparatus for vertically adjusting the fuel-air admission assemblies of windbox incorporating at least one such low load, split bucket fuel-air admission assembly wherein both the single bucket coal nozzles and the split bucket nozzles may be tilted in unison by the actuation of a single tilt actuation means at high loads in response to steam temperature, while still permitting the lower coal nozzles of the split buckets to be independently adjusted at low loads.